Crossfire is a multi-player graphical arcade and adventure game made for X. It has certain flavours from other games, especially Gauntlet and the rogue-like games (Nethack, Moria, Angband, and Ragnarok). Any number of players can move around in their own window, finding and using items and battling monsters. They can choose to cooperate or compete in the same world.

Arianne is a multiplayer online engine to develop
turn based and real time games. It provides a
simple way of creating games using Python for a
game's description. Marauroa, its server, uses
Java, MySQL, UDP, and Python for hosting dozens of
players.

Cyphesis is a server for MMORPG games. It is the core engine of the WorldForge project and is currently focused on implementing fantasy games, providing rich scripting, full world persistence, and basic physics.

AweMUD is a MUD server for use with fantasy-settings. Features include fully dynamic objects and characters, an advanced scripting system, and custom worlds. The engine will eventually have completely interactive rooms (items can be placed under benches, doors can be destroyed), complex magick, and interactive NPCs.

GNOME-Mud (formerly AMCL) is a Multi-User Dungeon (MUD) client that aims to make mudding a bit easier. It can define aliases, triggers, variables, keyboard shortcuts, and has a connection wizard for easy management of different player characters and MUDs.

The Interreality project is a collection of Open Source, Free Software projects based on the Virtual Object System (VOS) platform. The goal is to develop a free and open platform for multi-user 3D virtual reality games, applications, and interactive, collaborative 3D virtual spaces. VOS is a powerful and generic network object framework designed to support a variety of high-performance applications. It is the basis for Ter'Angreal, a general-purpose browser application for entering and participating in 3D worlds.

Deliantra is an advancement of the crossfire
multi-user RPG game. Unlike the original project,
it focuses mainly on stability and playability,
but also adds modern technology such as map
scripting, Perl server extensions, asynchronous
I/O, higher resolution graphics, and better client
support.